Research

Demographic heterogeneity and its effects on populations and communities

Population ecology has largely developed on the idea that important information is encapsulated in N, the total number of individuals, and that most characteristics of individuals (aside from age and size) can safely be ignored. However, individuals of the same age, size, and life stage may vary substantially in other phenotypic traits, some of which affect reproduction and survival. We are investigating when it is safe to treat a population as a collection of average individuals, and when the individual variability (which we call “demographic heterogeneity”) matters.

Our early work focused on small populations, and showed that demographic heterogeneity modifies the inherent variability of of these populations (“demographic stochasticity”), usually reducing it, and thereby reducing population extinction risk (Kendall and Fox 2002, 2003, Fox and Kendall 2002). Recent models focus on traits that an individual retains throughout its lifetime; such “persistent heterogeneity” can increase the long-term population growth rate (Kendall et al. 2011) and equilibrium density (Stover et al. in press) even in large populations. We are currently investigating how heterogeneity in dispersal affects invasion rate, and how heterogeneity affects community interactions (competition and predation).

We have applied these ideas to a population of the Florida Scrub-Jay (Aphelocoma coerulescens), a socially breeding bird that has been intensively studied at Archbold Biological Station. We found that there was substantial heterogeneity in survival (Fox et al. 2006), and, following the development of a model for demo-graphic stochasticity in reproduction (Kendall and Wittmann 2010), we are evaluating the magnitude of heterogeneity in reproduction and its effects on demographic stochasticity. Eventually we plan to integrate these effects into a model of population viability to understand how demographic hetero-geneity may help or hinder the long-term persistence of the population.

Population and community dynamics in a turbulent ocean

Many nearshore marine organisms have relatively sessile adults, dispersing as pelagic larvae and forming metapopulations. These larvae are advected by ocean flow, which at relevant time scales are highly turbulent (stirred not mixed), leading to high stochasticity in dispersal (Siegel et al. 2008). This makes empirical studies difficult, and has received little modeling attention. Starting from the Flow, Fish & Fishing biocomplexity project, we are studying how turbulent dispersal affects population structure and dynamics (White et al. 2010), competitive interactions and community structure (Berkey et al. 2010, Watson et al. 2011), and fishery management.